Papillomaviruses are small double-stranded DNA viruses that induce benign epithelial tumors or warts. These viruses are species specific and undergo their full life cycle only in their natural hosts. All papillomaviruses have similar reading frames, although differences are abundant in the amino acid sequences. Nearly 100 different human papillomavirus (HPV) genotypes have been distinguished. HPVs cause cutaneous, mucosal, and anogenital warts, and are associated with a high risk for the progression to cancer. Approximately 50% of all cervical cancers contain HPV-16 (Zur Hausen, Biochim. Biophys. Acta Rev. Cancer, 1288:F55-F78 (1996)). Other viral DNAs found in cervical cancer tissue include HPV-18, HPV-31, and HPV-33. HPV types 6 and 11 are frequently found in cervical papillomas yet are rarely associated with cervical cancer.